This invention generally relates to signal generating equipment. More particularly, the invention relates to apparatus and methods for generating continuous sampled waveform signals such as SONET signals of substantial length and complexity while utilizing a reasonable amount of data storage.
The telecommunications network servicing the United States and the rest of the world is presently evolving from analog transmission to digital transmission with ever-increasing bandwidth requirements. Fiber optic cable has proved to be a valuable tool of such evolution, replacing copper cable in nearly every application from large trunks to subscriber distribution plants. Fiber optic cable is capable of carrying much more information than copper with lower attenuation.
In attempting to accommodate the protocols, equipment, and cables of the past while providing for the direction of the future, various standards and system requirements relating to fiber optic cables have been adopted. In particular, the T1 Standards Committees of ANSI have provided a draft document ANSI T1.105-1988 dated Mar. 10, 1988 which sets forth specifications for a rate and format of signals which are to be used in optical interfaces. Additional details and requirements are set forth in Technical Advisory publications SR-TSY-000202, --000233, -000253, -000303 Issue 3 of Bell Communication Research (Bellcore). The provided specifications detail the SONET standard. SONET defines a hierarchy of multiplexing levels and standard protocols which allow efficient use of the wide bandwidth of fiber optic cable, while providing a means to merge lower level DS0 and DS1 signals in a common medium. In essence, SONET establishes a uniform, standardized transmission and signaling scheme which provides a synchronous transmission format that is compatible with all current and anticipated signal hierarchies. Because of the nature of fiber optics, expansion of bandwidth is easily accomplished.
The data structure of a basic SONET signal (data rate 51.84 Mbps) termed an STS-1 signal, is seen in FIG. 1 and has nine rows of ninety columns of eight bit bytes at a 125 microsecond frame period. The first three columns of bytes in the SONET signal are termed the transport overhead (TOH) bytes and are used for various control purposes. The remaining eighty-seven columns of bytes constitute the STS-1 Synchronous Payload Envelope (SPE). Included in the overhead bytes are "pointer" bytes which indicate to the receiving equipment where a block of data begins within the data payload. The ability to test the operation of receiving equipment in response to changes in pointers is highly desirable.
Another SONET signal which is at three times (155.52 Mbps) the data transmission rate of the STS-1 signal is an STS-3 signal. The STS-3 signal has nine rows of two hundred and seventy columns of eight bit bytes with the first nine columns of bytes being TOH bytes.
As more and more SONET signal equipment (transmitting, receiving, and switching) is being designed and utilized, a need has arisen for apparatus which generates SONET test signals which can be used to test new equipment. A primary limitation of the apparatus of the art which has been provided to fill the need for SONET-testing equipment has been the cost of the memory required. For example, to store an STS-1 test sequence of one second in length, approximately six and a half (6.5) megabytes of memory are required. For a one second STS-3 signal, approximately nineteen and a half (19.5) megabytes of memory are required. In order to limit memory costs, the test equipment apparatus of the art have limited themselves to extremely simple sequences which require limited memory. However, such simple sequences are not capable of permitting noise measurements which require longer sequences of several, if not many, seconds in length. Nor are such simple sequences capable of testing and monitoring the movement of the control structures, such as pointers within the SONET signal, as such testing and monitoring likewise requires long sequences.